The present invention relates to a method for actuating a reversible belt pretensioner.
Seat belts which are nowadays usually installed in motor vehicles have a mechanism for automatically winding the loose belt webbing onto a belt winding-on roller which is arranged on a belt reel. The automatic winding on ensures that the belt which is being worn rests loosely against the body of a vehicle occupant and is rolled up on the belt winding-on roller when the seat belt is not in use. The belt is unwound from the belt winding-on roller counter to a torque which is generated by a retraction spring, and this is also possible when the belt is being worn, in order to permit the occupant to make a largely free movement.
Furthermore, there is usually a locking device which acts as a belt extraction lock. This locking device is triggered by an actuation device with a belt-webbing-sensitive and vehicle-sensitive sensor, when the belt is being unwound quickly from the belt winding-on roller, starting from a predefined acceleration of the belt winding-on roller in the unwinding direction, or starting from a predefined acceleration of the vehicle. As a result, the belt winding-on roller is secured in the position in which it is in at this time, so that the belt webbing is prevented from unwinding. A customary locking device is composed, for example, of a locking toothing of the belt reel, and of a locking latch, which is pivotably mounted and can be pivoted into the locking toothing of the belt reel by means of the actuation device. The toothings on the belt reel and the locking latch are of self-locking design, causing the extraction of the belt to be locked for as long as tensile loading is applied to the belt. If the belt is relieved of this tensile loading, the locking mechanism can be opened, i.e. it can be returned into its inactive state. This return can be carried out magnetically, electromotively or, usually mechanically, for example by a restoring spring on the pivotably mounted locking latch.
In addition, in many seat belts there is a belt pretensioner which increases the protective effect of the seat belt. The belt pretensioner has a drive which can be actuated in order to drive a mechanical device which shortens the loose belt webbing and/or draws an occupant rearward. In the case of a pyrotechnic energy store, for example, a chemical substance is made to undergo an exothermic reaction using a firing element. In this reaction, a gas stream is generated which drives the mechanical device. The driven device is mechanically connected, or can be mechanically connected, to the belt reel, for example by a clutch. Via this connection, the mechanical device exerts a torque on the belt reel. Owing to this torque, the belt reel rotates with the belt winding-on roller arranged on it and pretensions the unwound belt webbing.
In addition to the belt pretensioners, generally pyrotechnic belt pretensioners which are used nowadays in motor vehicles, use can also be made in vehicles of reversible belt pretensioners which can be triggered repeatedly, even in rapid succession. These reversible belt pretensioners can have different drives; for example, a belt pretensioner of this type can be driven by an electric motor which acts permanently or in a controllable manner via a clutch on the belt reel. Other reversible belt pretensioners are driven with compressed air from a pressure accumulator, or by a stressed spring, with it being possible to refill the pressure accumulator during the travel mode and to restress the spring during the travel mode.
Reversible belt pretensioners which are driven in this manner permit the seat belt to be pretensioned to a predefinable degree, at a predefinable speed and for a predefinable period of time. The multiple triggering capability of the reversible belt pretensioner permits it to be triggered preventively. Preventive triggering allows the belt pretensioner to be triggered in safety-critical travel situations which are detected, for example, be vehicle-movement dynamics sensors or vehicle-surroundings sensors or which are determined by the evaluation of the actuation of the brake pedal, of the steering angle or of a driver observation.
In addition to preventive triggering, a reversible belt pretensioner can also be used for tactly warning the driver in safety-critical situations. When there is preventive triggering of the belt pretensioner, which may take place before the detection of a collision, or when the belt pretensioner is triggered for warning purposes, it is desirable that, after pretensioning has taken place, after the end of the hazardous situation and with the normal travel mode safeguarded, the belt rests loosely against the occupant again. A normal travel mode is safeguarded if the evaluation of the situation by a control unit or a hazard computer does not reveal that there is a safety-critical situation or if the condition necessary for triggering the belt pretensioner is no longer satisfied. Such a condition may be the presence or the absence of a certain signal on a data bur or a data line.
After reversible pretensioning of the seat belt with a reversible belt pretensioner, it is possible that the belt extraction lock has become active owing to a belt-webbing-sensitive sensor or a vehicle-sensitive sensor which actuate the locking device electrically or mechanically. Examples of belt-webbing-sensitive sensors are a mechanical centrifugal-force sensor in the belt winding-on mechanism, an electromechanical centrifugal-force sensor or an electronic belt extraction sensor which senses the extraction speed of the belt webbing or its acceleration.
A belt-webbing-sensitive sensor can respond, in particular, if, after belt pretensioning, the pretensioned belt, which is under a tensile loading, is released. This tensile loading of the pretensioned belt is, in particular, dependent on the strength of the pretensioning process which has previously occurred and on the sitting position of the occupant before the pretensioning process. As a result of the tensile loading, after a pretensioning process the belt is unwound again from the belt winding-on roller after the torque exerted on the belt reel by the belt pretensioner drive decreases. If the unwinding of the belt webbing takes place too quickly, the belt-webbing-sensitive sensor responds and the locking device is actuated, generally mechanically, so that it closes. This means that the belt extraction lock becomes active.
The belt extraction lock can also become active already if the belt-webbing-sensitive sensor responds owing to the winding-on process during the pretensioning of the belt, or the vehicle-sensitive sensor responds owing to the movement of the vehicle. If the belt extraction lock is active, it is no longer possible to unwind the pretensioned belt and the freedom of movement of the occupants is greatly restricted. The belt webbing is therefore to be released again if the cause of the triggering of the belt pretensioner no longer applies and/or if a normal travel mode is safeguarded. In order to release the belt webbing again, in the case of the locking devices which are customary nowadays in motor vehicles, the locking latch is to be released from the locking toothing. The release of the belt webbing is only possible if the vehicle-sensitive and the belt-webbing-sensitive sensor do not actuate the locking device in order to lock the same.